Learning ACAD, at least for me was a doit, discover, (forgot how), doit again process.
Very powerful. Throw lots of time at it. I've only ever used the help and I do complex stuff in 3D.
Hardest part is getting the 2D paper result done properly with the least amount of work from the 3D models.
For final drawings (2D) make a separate .dwg file for the 3D part, use copy with basepoint, from your assemby 3D file, to get the latest model.
Eventually too many layout tabs become totally confusing and un-manageable.
Maybe OK from assemblies in inventor, but not in ACAD. I don't use inventor. If you learn the ACAD principles, the step to inventor will be easy. Not so the other way.
Also time consuming learning how to import/export to other packages.
When dealing with other people, you will often find they use something other than ACAD and if you want 3D models from them, you need to figure out how to get something that ACAD understands.
Dealing with Chinese, Pro/e, and maybe Solidworks seem to be popular.
The only way I have found to import reliably from these is to use Mastercam 9 to import, then export to ACIS format (version 6).
Keep hitting keys, and make sure you end up with exact sizes for what you are doing, and just not fudgy numbers by doing too much visually with the mouse (random input device that never does the same thing ever again.)
For instance. Start to draw a line from somewhere, use the mouse to give the direction, then from the command line enter a length, followed by a space. (not enter, because, when you start to use text, you will go nuts because enter does not terminate the command)
Learn to manipulate UCS - most important part to learn.
Learn to use snap (F3) key and ortho(F8) key all the time.
Use numbers at bottom of screen (F6)